Conventional electrohydraulic servo valve assemblies and valve/actuator systems embody a number of non-linearities in the output versus valve control signal characteristic. For example, some valves embody so-called overlap which manifests itself as a flat spot near the origin of the flow versus control signal curve. Valve null shift near the curve origin is another common source of error. Valve non-linearities away from the origin can result from malfunction, such as a loose valve ball, either initially or after a period of operation. Significant non-linearities can also be caused by the load to which the valve is coupled, such as differential area actuators, or by flow saturation due to limited pump output.
It is conventional practice to attempt to overcome design non-linearities such as overlap and null shift through complementary electronic compensation. Thus, a specific analog control circuit may be designed to complement a specific valve or a specific valve/actuator systems. Such control circuits do not possess desired versatility and are difficult to adjust in the field. Moreover, a change in operating characteristics at the valve or load, due to environment or malfunction, leads to error and/or requires readjustment. A valve malfunction, such as spool winding or a loose ball in the valve servo spool ball hole, a problem known as "ball glitch", cannot be readily compensated in the electronics, and thus typically requires valve repair or replacement.
Another problem in the art lies in the requirement that diagnosis of a valve or valve/actuator problem usually involves hands-on testing by a skilled technician. Thus, a technician must travel to the location of use, or the valve must be replaced and shipped to the technician for service. Both options are expensive and time-consuming.
A general object of the present invention, therefore, is to provide a system and method for measuring and/or compensating valve flow characteristics which alleviate or overcome the aforementioned deficiencies in the art. More specific objects of the invention are to provide a system and method for deriving flow characteristics of a servo valve in a valve/actuator system which may be readily and economically electronically implemented, and which may be carried out remotely from the site of use without requiring disconnection of the valve and/or actuator. A further specific object of the invention is to provide a system and method of the described character which automatically compensate for non-linearities to yield a linear actuator velocity versus command signal characteristic.